The present invention relates to an image signal processing device for correcting irregularities or degradation of MTF characteristics of an image signal due to a reading optical system, using an image sharpness emphasizing process.
There is known an image scanning reading system in which image information of an original or copy is scanned with a reading optical system to produce an image signal representing the image information.
FIG. 1 of the accompanying drawings shows such an image scanning reading system in which image information carried on an original or copy A is scanned in a main scanning direction indicated by the arrow X with two line sensors 2, 4 while the original A is being fed in an auxiliary scanning direction indicated by the arrow Y. Light reflected from the original A between scanned positions a.sub.1, a.sub.2 along a main scanning line 6 is guided by a fully reflecting mirror 8 and a lens 10 to the line sensor 4, by which the light is converted into an electric signal. Light reflected from the original A between scanned positions a.sub.2, a.sub.3 along the main scanning line 6 is guided through the fully reflecting mirror 8, the lens 10, and a fully reflecting mirror 12 to the line sensor 2, by which the light is converted into an electric signal. The electric signals produced by the line sensors 2, 4 are combined into an electric signal which corresponds to the image information borne by the original A between the scanned positions a.sub.1, a.sub.3 along the main scanning line 6.
The image scanning reading system has the problem of MTF (Modulation Transfer Function) irregularities of an image signal due to the reading optical system which includes the fully reflecting mirror 8, the lens 10, the fully reflecting mirror 12, and the line sensors 2, 4.
For example, as shown in FIG. 2, the MTF characteristics are degraded in the vicinity of an area where the line sensors 2, 4 are optically joined, the area corresponding to the scanned position a.sub.2 on the original A. This MTF degradation is caused by the fact that the opening of the lens 10 as viewed from the line sensor 4 is vignetted by the fully reflecting mirror 12, as shown in FIG. 3. As a result, the MTF characteristics are degraded in the area where the line sensors 2, 4 are optically joined.
The MTF irregularities with respect to the scanned position x are also caused by different characteristics of the line sensors 2, 4 or the angle of view (or the image height) of the lens 10.
With the MTF characteristics of the image signal being rendered irregular due to the reading optical system, when a striped highlight image as shown in FIG. 4(a) is read by the line sensors 2, 4 which are optically overlapped as shown in FIG. 4(b), the line sensors 2, 4 produce an image signal as shown in FIG. 4(c). If a slicer level 14 is selected with respect to the image signal as shown in FIG. 4(c), then an image reproduced from the generated image signal looks as shown in FIG. 4(d). When a striped shadow image as shown in FIG. 5(a) is read by the optically overlapped line sensors 2, 4 as shown in FIG. 5(b), the line sensors 2, 4 produce an image signal as shown in FIG. 5(c). When a slicer level 14 as shown in FIG. 5(c) is used with respect to the image signal, a reproduced image looks as shown in FIG. 5(d).
The MTF irregularities as described above bring about density irregularities in reproduced images when image signals are processed. When a continuous-tone image is read from the original A and converted into a halftone-dot image, not only image density irregularities are produced, but also the sharpness of the image is made irregular due to the MTF irregularities.
It is known that the MTF irregularities depend on not only the scanned position x on the original A, but also the scanning direction as shown in FIG. 6. Specifically, the MTF characteristics vary between the main scanning direction indicated by the arrow X and the auxiliary scanning direction indicated by the arrow Y. Accordingly, the sharpness of the reproduced image varies depending on the scanning direction.
The image signal produced by the image scanning reading system also suffers a problem in that the MTF characteristics are degraded as the spatial frequency of the digitized image goes higher. If the MTF characteristics are degraded depending on the spatial frequency of the digitized image, then the sharpness of the reproduced image is also degraded.
In order to compensate for such degradation of the image sharpness, there has been proposed a signal processing system for effecting a sharpness emphasizing process on an image signal.
However, since the MTF characteristics are degraded as the spatial frequency of the applied digitized image, it is highly difficult and requires a highly complex signal processing system to carry out sharpness emphasis over a wide range of spatial frequencies.